1. Technical Field
The present invention relates to iron casting in a manner which produces a unitary workpiece having different material properties in different zones of the casting, with such differing properties being produced during the molding process.
2. Description of Related Art
Grey iron has been used in castings since the dawn of the automotive age. Iron is inexpensive and is relatively easy to cast. It does, however, suffer from comparison of its strength and stiffness with the characteristic of other materials such as steel. Although it has been known to increase the cross sectional thickness of say, a cylinder block, in order to improve engine durability and noise, vibration, and harshness characteristics, this is a costly remedy for the deficiencies of grey iron and causes a weight penalty which, those skilled in the art will appreciate, is an almost intolerable situation in the automotive business today. Those skilled in the art appreciate that grey iron, when properly doped with manganese to compensate for sulfur contained in the iron, may be cooled in such a fashion so as to produce compacted graphite iron or nodular iron. It is known conventionally to produce nodular iron gear cases for the center section of high performance rear drive vehicles, and for engine crankshafts and other highly stressed automotive components. This works quite well when the machining process is limited almost entirely to grinding. Nodular iron is, however, difficult to machine and it is more desirable to use compacted graphite iron for machinability reasons, while attaining a reasonable level of strength and stiffness. However, in the absence of the present invention, it has not been known to use preferential cooling rates to achieve both nodular and compacted graphite iron in a single casting.